Study On The Triassic-jurassic Tectonic Evolution Of The Middle Great Xing'an Range

Great Xing'an Range, the core portion of Northeast Asian region that witnessed the influence of the various periods and tectonic domains, is a key to study the orogenic evolution history in the Northeast Asian and the Central Asia. Most recent researches focus on Late Paleozoic and Late Mesozoic. However, little has been reported about Triassic and Jurassic, especially about the closure of the Paleo-Asian Ocean and the magmatite activity. Isotope tests indicate that most of the volcanic activities are despite generally concern on the late Jurassic volcanic rocks concentrated in the Early Cretaceous. The reason why the middle Great Xing'an Range has been chosen as the study region is that the outcrop area of Mesozoic Strata is quite complete in this area.Based on the combination of the field data and the latest results published in the study region that, this paper rebuild the original tectonic evolution of Triassic-Jurassic generally. And the following understanding is achieved.Hadataolegai Formation adakites in early Triassic are identified. Paleo-Asian Ocean closes along the line of Silumulon River-Changchun-Yanji which collision activated Hegenshan-Heihe structural belt. The stress push the young crust to underthrust from south to north along Hegenshan-Nenjiang-Heihe line. The lower crust or the plate remain are advanced to the deeper mantle melting and then the adakites are formed. What's more, lacustrine clastic sediments are deposited in the surface intermountain basin. Therefore, the closure of Paleo-Asian Ocean is a kind of continental subduction that the line along the of Silumulon River-Changchun-Yanji is the main collision belt while the line along Hegenshan-Nenjiang-Heihe is activated structural belt internal block. The former is earlier than the latter and larger than the latter in scale.2,Both post-orogeny post-collision granites of middle-late triassic widely developed and A2type granite in partial area indicates that the crust of transforms compression into extension under the tectonic setting. Meanwhile, Chaihe group molasse of Upper Triassic which marks the termination of orogeny is determined by paleontology and isotopic dating methods. Therefore, Ancient Asian Ocean tectonic domain affects the study area extended to the Late Triassic at least.3,The coal-bearing construction deposition early Jurassic in the study area of the main Red Flag Group and Manpower Group shows that this period in the study area is under the tectonic background of regional extension stable fault sedimentary. Comparing volcanic rocks of the middle Jurassic Tamulangou formation in the working area with the northern section of Great Xing'an Range and Root River region, conclusion can be drawn that the middle-late Jurassic in Great Xing'an Range has subduction polarity characteristics from north to south which is the reflection Mongolia-Okhotsk subduction geodynamic.4,Identify a large thrust nappe structure in the study area, its structure, deformation and formation period were studied. In summary, comparing thrust nappe structures in study area with peripheral areas, it was occurred by the Siberia plate moving to south and the North American plate sinistral promoting in Late Jurassic when Mongolia-Okhotsk Ocean eventually closed in Mohe line. A large nappe caused by the collision the entire Northeast region and the coarse clastic syntectonic sedimentation is widely developed.